Tapping mode atomic force microscopy (TMAFM) is a widely used dynamic imaging technique that maps surface topography by monitoring the oscillation amplitude of a cantilever integrated with an ultra-sharp tip probe, driven by a piezoelectric bimorph element mounted at the cantilever root, as described in several issued US patents. In this imaging mode, the cantilever is commonly driven near its resonance frequency ωo, and the intermittent tip-sample contacts lead to the decrease of cantilever oscillation amplitude from the “free” amplitude Ao to tapping amplitude A. The sample surface acts as a repulsive barrier that limits the tapping amplitude of the cantilever. For a rigid surface, this decrease of cantilever oscillation amplitude is linear with the decrease of the distance between the tip and the sample Do. Thus, the surface topography can be tracked by rastering the tip in the xy plane and using a feedback loop to continuously adjust the vertical (z) extension of the piezoelectric scanner to maintain the constant set-point s=A/Ao. The technique of scanning probe acceleration microscopy (SPAM) builds upon current proximal probe technology and instrumentation. Such techniques include jumping mode AFM, tapping mode AFM, fluid tapping mode AFM with a replaceable fluid cell excited acoustically and magnetically, and other variations of scanning probe microscopy with cantilever driven near its natural frequency.
There is considerable interest in using tapping mode AFM to study elastic and viscoelastic mechanical properties of surfaces, which would be beneficial in enhancing the ability to characterize materials and map mechanical and/or chemical variations of surfaces at the nanoscale in a much gentler fashion. Such information would be particularly useful in elucidating changes in biologically relevant surfaces such as lipid bilayers, cell surfaces, and other biomacromolecular complexes exposed to various factors. This could be particularly useful in elucidating potential effects of beta amyloid (a peptide implicated in Alzheimer's disease) or other peptides (such as those associated with conformational disease) on such surfaces that may modulate their mechanical properties. Much of this information can be ascertained from the time-resolved force interaction between the surface and tip, but currently there is not a straightforward manner to obtain these force trajectories in tapping mode. In the absence of such a straightforward technique, the phase of the cantilever in tapping mode is commonly used to glean some information about the mechanical properties of surfaces; however, multiple sources of energy dissipation (i.e. capillary forces, viscoelasticity of the sample, cross talk with topography, etc.) make it difficult to interpret phase images. There is also a significant contribution due to frictional forces associated with the tilt of the cantilever and/or surface.
A more complete insight into the mechanical properties of the sample can be obtained by deeper analysis of the cantilever deflection trajectory involving studying its higher harmonic content. When the harmonic drive signal is applied to the cantilever, the resulting oscillation is also harmonic. When the tip taps the surface, the harmonic motion of the cantilever is distorted at the bottom of each oscillation cycle, resulting in anharmonicity, which shifts a certain amount of power to higher harmonics. In traditional TMAFM, which monitors cantilever deflection only at the oscillation frequency, information about anharmonicity is lost. The easiest way to retain it is by digitizing the entire cantilever trajectory at sufficiently high frequency (at least twice the frequency of the highest harmonic) and high bit resolution. Recent developments in the area of high-speed A/D converters make this task entirely possible. When using higher harmonics to reconstruct the tip-sample force interaction per oscillation cycle, the transfer function of the cantilever detection system must also be known, and this can be difficult to obtain. Currently, the most straightforward method of measuring the transfer function of a cantilever involves studying the oscillation decay of a cantilever subjected to an initial deflection. The initial deflection can be provided by running a force curve experiment on a strongly adhesive surface. It must also be noted that there are other sources of anharmonicity in cantilever deflection signals. These include nonlinearities of the detector and electronics of the AFM. Also, higher eigenmodes of the cantilever can complicate analysis based on higher harmonics. Based on this, efforts have been made to produce cantilevers with well defined eigenmodes to enhance the higher harmonic content in tapping mode AFM signals. Another common problem in analyzing higher harmonics is the rapid decay of the harmonic envelope, which can effectively place them below the noise level.
Accordingly, there is a need for improved scanning probe acceleration microscopy. Those and other advantages of the present invention will be described in more detail hereinbelow.